In recent years, by rapid progress of an MEMS technology (Micro-Electro Mechanical systems), a development of a small thin film-movable element for electrically displacing or moving a small thin film of μm order has intensively been carried out. As the small thin film-movable element, there is, for example, a digital micromirror device (DMD) for deflecting light by inclining a micromirror, an optical switch for switching an optical path or the like. DMD has a wide use in a field of an optical information processing such as a projecting display, a video monitor, a graphic monitor, a television set and electrophotography printing and so on. Further, an optical switch is expected to be applied to optical communication, optical interconnection (signal connecting technology by light such as an intercoupling network in parallel computers), an optical information processing (information processing by optical operation) and the like.
Generally, a small thin film-movable element includes a movable portion elastically deformably supported and capable of being displaced one-dimensionally or two-dimensionally, and the movable portion mainly deals with switching operation. Therefore, it is particularly important to control the braking of the movable portion in excellently carrying out the switching operation.
For example, a micromirror apparatus disclosed in JP-A-8-334709 is constructed by a constitution of applying a voltage to one electrode of a pair of drive electrodes and rotating a movable portion having a mirror arranged between the electrodes by hinge connection by an electrostatic attractive force in accordance with a potential difference and an electrostatic capacitance between the movable portion and the drive electrodes.
Further, according to a method of attenuating a ribbon element in a small mechanical lattice apparatus disclosed in JP-A-2001-174720, in a method of determining a bottom surface and attenuating an electromechanical ribbon element above a channel having a bottom conductive layer formed below the bottom surface, there are a step of providing at least one constant amplitude voltage pulse to at least one ribbon element, and a step of providing a brake pulse divided from the constant amplitude voltage pulse by a narrow temporary gap to at least one ribbon element. That is, the electrostatic force is operated in a single direction by one movable portion electrode of a parallel flat plate system element and one fixed electrode, and vibration is controlled by two of brake drive voltages of a drive voltage for attracting the ribbon element to a side of a lower electrode, and an initial brake voltage applied immediately before the drive voltage, or a final brake voltage applied immediately after the drive voltage.
Further, an optical path switching apparatus disclosed in JP-A-2002-169109 includes a mechanical optical switch for switching an optical path by applying a signal voltage to an electromagnetically driven actuator, and a control circuit for supplying the signal voltage to the optical switch. According to the signal voltage, with regard to a rise amplitude VH and a signal width T of the signal, a voltage after an elapse of T/2 from rise of the signal is equal to or smaller than ⅔ VH. Further, vibration is intended to restrict by reducing the signal voltage equal to or smaller than ⅔ times as much as a rise amplitude when T/2 has elapsed from rise of the signal of the signal width T in the signal voltage applied to the actuator.
Further, a method of controlling a micromachine element disclosed JP-A-2002-36197 is constituted such that a first control signal and a second control signal are supplied to the micromachine element, the second control signal sets the micromachine element to an active state, and the first control signal maintain the state. Further, the micromachine element is controlled by using at least two of control signals of a control signal for setting the micromachine element to a pull-in state and other control signal for maintaining the micromachine element to the pull-in state. Thereby, the micromachine element can be controlled at a low voltage level.
Further, according to a method of controlling to switch an optical switch disclosed in JP-A-2-7014, in an optical switch including a vibrating member displaced by making a control voltage ON, OFF, and an element provided at a front end of the vibrating member for reflecting or cutting propagated light by displacing the optical element, before making the control voltage ON, a first preliminary voltage pulse shorter than a period of a natural frequency of the vibrating member is applied to the vibrating member, after making the control voltage OFF, a second preliminary voltage pulse shorter than the period of the natural frequency of the vibrating member is applied to the vibrating member.
Generally, in an optical switch, when a vibrating member is displaced by making a control voltage ON, OFF, a phenomenon referred to as chattering is brought about. The chattering is a phenomenon in which after making a control voltage ON or OFF, the vibrating member is not changed immediately by an amount of displacement in correspondence with the control voltage but is displaced finally by the amount of displacement in correspondence with the control voltage while carrying out a large attenuating vibration. Therefore, until an optical output becomes a constant level by attenuating the vibration, an optical path is not switched and a speed of switching the optical switch is restricted. In contrast thereto, according to the method of controlling to switch the optical switch by the background art, by applying the preliminary voltage pulse shorter than the period of the natural frequency of the vibrating member before making the control voltage ON and after making the control voltage OFF, chattering is controlled and the speed of switching optical switch is increased.
Further, according to a small electromechanical modulating element disclosed in JP-A-2001-242395, there is provided a small electromechanical modulating element array capable of actively reducing a vibration of a movable portion to achieve high speed formation of switching operation.
However, according to the micromirror apparatus disclosed in JP-A-8-334709, the voltage is applied to one of the drive electrodes, the electrostatic attractive force in accordance with the potential difference and the electrostatic capacitance between the movable portion and the drive electrodes is generated to thereby rotate the movable portion. Therefore, as shown by FIG. 24A, a vibration is generated by receiving a repulsive force from a contact member immediately after the micromirror is moved to a contact position to be grounded on the contact position by applying a voltage Va. Further, even in a case of a noncontact structure in which the micromirror is not grounded on the control position, as shown by FIG. 24B, a time period is required until converging the vibration by bringing about overshoot by exceeding a desired rotational angle (converging position). The vibration or the overshoot hampers high speed formation in switching operation of the small thin movable element.
Further, according to the small mechanical lattice apparatus disclosed in Patent JP-A-2001-174720, as shown by FIG. 25A, a constant amplitude voltage pulse 1 is a function of time and is provided with a duration time period of 2 μsec and a voltage value of 10 V. Immediately after the constant amplitude voltage pulse 1, a narrow brake pulse 5 divided from the constant amplitude voltage pulse 1 by a narrow gap 3 is applied. Further, when constant amplitude voltage pulses 7 contiguous to each other are provided with inverse polarities as shown by FIG. 25B, also a polarity of the brake pulse 9 becomes inverse to that of the voltage 7 related thereto. However, the small mechanical lattice apparatus is a small thin film-movable element of a so-to-speak parallel flat plate type for displacing the ribbon constituting the movable portion to a side of a board in parallel therewith, a pulse is applied to one movable portion side electrode and one fixed electrode opposed thereto to brake and therefore, there is a disadvantage of being devoid of a variety in the vibration control method. For example, when the movable portion is displaced by being attracted to the board, a brake force in a reverse direction cannot simultaneously be applied. That is, vibration cannot actively be reduced.
Further, according to the optical path switching apparatus disclosed in JP-A-2002-169109, in the electromagnetically driven actuator, when the movable portion becomes proximate to a front end of a yoke, that is, when an attractive force by a magnetic field of a permanent magnet becomes intensive, an attractive force by a magnetic field of a coil is reduced and the movable portion is moved to a position of connecting a fiber such that a synthesized attractive force does not become excessively strong. A waveform outputted by a signal generating circuit is a signal voltage in which the voltage rises by a rise voltage VH=7 V and is thereafter rapidly dropped as shown by FIG. 26A. The signal width T is 5 ms, and a voltage at a terminal end of the signal is 0.5 V. A voltage after an elapse of T/2 from rise is 0.8V. In FIG. 26B, a rise voltage is 7 V, the signal width T is 5 ms, and a time period To of changing the amplitude in a step-like shape=1.5 ms. In FIG. 26C, the rise voltage is 5 V and the time period T until the reduced amplitude becomes 1 V=2 ms. The time period T corresponds to the signal width. The voltage of 1 V is continued to be applied until carrying out next switch after the time period T. FIG. 26D shows a waveform in which the rise voltage is 5 V, the time period To until the step-like amplitude is changed=3 ms and the amplitude is changed in the step-like shape by a constant value 0.5 V. The signal voltages can restrain chattering by accelerating movement (switching speed) of the movable portion by increasing the amplitude of rise and reducing a force applied to the movable portion by rapidly reducing the signal voltage when the movable portion is started to move. However, although according to the optical switching apparatus, a block constituting the movable portion is bidirectionally being displaced in parallel, the vibration is intended to be restrained by changing the drive force operated in a forward direction and therefore, there is a disadvantage of being devoid of a variety in the vibration control method. Further, basically, the attractive force by the magnetic field of the coil is reduced, the signal voltage is reduced such that the synthesized attractive force does not become excessively strong, similar to the above-described, vibration cannot actively be reduced.
Further, according to the method of controlling the micromachine element disclosed in JP-A-2002-36197, the control is carried out by using a single or a plurality of control signals. Typical waveforms of the control signals are shown by FIGS. 27A to 27H. As is known from FIGS. 27A and 27B, the control signal may be constituted by a pulse row for changing the state of the micromachine element. Similarly, in the case of at least two control signals, the signals may be signals synthesized in superposed signals drawn in FIGS. 27C and 27D, an amplitude modulating (AM) signal shown in FIG. 27E, a frequency modulating (FM) signal shown in FIG. 27F, a pulse width modulating (PWM) signal drawn in FIG. 27G, or a pulse density modulating (PDM) signal drawn in FIG. 27H. However, according to the control method, an object of control is constituted by reducing a voltage maintained in the pull-in state, reducing ON/OFF delay by discharging residual charge, increasing the amplitudes of the output signal and the like, and vibration cannot actively be reduced.
Further, according to the method of controlling to switch optical switches disclosed in JP-A-2-7014, as shown by FIG. 28, before making the control voltage ON, OFF, the first preliminary voltage pulse, the second preliminary voltage pulse are applied to the vibrating member, the electrostatic force is operated in a signal direction by one movable portion electrode and one fixed electrode, and vibration in driving the movable portion is restrained by a balance of an elastic force and an inertia force of a movable support portion, the electrostatic force (potential difference) only in the forward direction of being operated in the movable transiting direction is changed and therefore, an effect of restraining vibration is small. Generally, in an optical switch for optical communication, different from DMD, much time period is taken until converging free vibration for being positioned at an arbitrary angle. Further, optical information of laser light or the like is reflected to be incident on a fiber on an emitting side and therefore, a high control accuracy is requested, vibration of the movable portion (mirror portion) constitutes a cause of noise as the above-described chattering. In this way, particularly in the case of the optical switch, the influence of vibration is larger than that of DMD, which is desired to be improved.
Further, the small electromechanical modulating element disclosed in JP-A-2001-242395 comprises a mirror 13 attached to a yoke 12 by a mirror support post, a mirror address electrode 14, a yoke address electrode 15, and a catch electrode 16. According to the small electromechanical modulating element, the catch electrode 16 is provided at a pertinent height position above a surface of the element to be proximate to a continuous end portion of the mirror 13 or the yoke 12 when the mirror 13 is rotated in a desired angle region. The catch electrode 16 and the mirror assembly generate an extremely high electrostatic attractive force therebetween, and when the mirror becomes proximate to a plane of the catch electrode, the mirror is biased to stop the mirror 13. Therefore, by controlling pulse waveform of the mirror assembly, the mirror 13 is firmly attenuated, and vibration of the DMD mirror is prevented when the mirror assembly is stopped at a desired rotation angle. However, the small electromechanical modulating element is provided with a drawback that an effect of restraining vibration is small since vibration is intended to restrain by noncontact drive. Particularly, the catch electrode 16 for restraining vibration of the yoke 12 in being operated by a physical acting force is arranged on a plane the same as that of the yoke 12, and is braked by a physical acting force between end faces thereof and therefore, areas of the electrodes opposed to each other are small, a large physical acting force is not achieved and an efficiency of restraining vibration is poor.